This invention is directed at the separation of a crystallized component from a slurry. More specifically the subject invention is directed at the separation of high melting point refined wax from a slack wax feed stream.
In the production of lube oils and waxes it is important to effect a good separation of the lube oil from the wax. The presence of wax in lube oil adversely affects the pour point of the oil, while the presence of lube oil in wax is not desirable for several reasons. Since the unit price of lube oil products typically is higher than that of wax products, eonomic considerations dictate that the oil content of the wax be as low as possible. In addition, many refined wax products, such as those used in contact with food, require that the residual oil content be maintained below a predetermined value. The removal of oil from wax typically has involved the use of a chilling zone to precipitate the wax, solvent addition to remove some of the residual oil from the wax, and a separation zone to remove the wax from the oil and solvent. Where the oil content of the wax is not reduced sufficiently in one separation stage, it may be necessary either to further process the wax or to utilize the wax in lower quality applications. Reprocessing the wax, such as by passing the wax through one or more separation zones, may not be desired because of the additional operating and capital costs. Previous work has been directed at the separation of lube oil from wax. U.S. Pat. No. 4,146,461 is directed at the dewaxing of waxy lubricating oil stocks by the injection of cold dewaxing solvents at a plurality of points. The patent discloses adjusting the cold solvent addition rate to each stage to ensure that the temperature drop in the initial stages is greater than the temperature drop in the final stages. U.S. Pat. No. 3,644,195 is directed at the separation of a waxy oil stream by adding cold solvent to a multi-stage mixing zone to crystallize the wax. The wax, separated from the lube oil by rotary filters, is again mixed with solvent at a temperature sufficient to dissolve low-melting wax only, after which the high-melting wax is separated by another rotary filter.
U.S. Pat. No. 2,284,607 is directed at a method of dewaxing oil. This patent discloses the chilling of the primary solvent and feed stream mixture and the subsequent addition of a secondary solvent at a higher temperature than the primary solvent-feed mixture. After the secondary solvent is added, the mixture again is chilled, after which the wax is separated.
U.S. Pat. No. 4,169,039 also is directed at dewaxing an oil. This patent discloses the use of a multi-stage mixing and crystallization zone in which relatively small amounts of the components from the hot washing drum are recirculated to the mixing zone, but at a lower temperature than the material being processed in the mixing zone.
In all of the patents noted above the separation of the oil from the wax requires the use of additional quantities of solvent and/or additional processing steps. Accordingly, it is desirable to provide a process which will reduce the residual oil content in wax to relatively low values without the use of additional processing equipment or additional processing steps.
It also is desirable to provide a process which will reduce the residual oil content in wax to relatively low levels without the use of excessive amounts of solvent.
It also is desirable to provide a process which will permit a decrease in the wash solvent addition rate without increasing the residual oil content of the wax above a predetermined limit.
The subject invention is directed at a method for separating a first, crystallized component from a second, non-crystallized component by passing the feed stream comprising the first and second components through first and second mixing zones. Solvent is added to both mixing zones, with the temperature of the solvent added to the first zone lower than that added to the second mixing zone. More specifically, the subject invention is directed at reducing the residual oil content of a wax fraction by passing the wax-containing feed stream through a first mixing zone where the feed stream is contacted with a solvent at a lower temperature than the feed stream to precipitate the wax and form a wax slurry. The slurry is then contacted in a second mixing zone with solvent at a higher temperature than the solvent added to the first mixing zone to remove residual oil from the wax fraction. The slurry exiting from the second mixing zone is passed to a separation zone for separation of the wax fraction from the slurry.